I'd suggest using the paper file pages which are available. These allow the negative strips to 'breathe' far more than the plastic sleeves. The paper may well be less protective than the plastic in the long term (many decades), but I have been using both sorts of sleeves for nearly forty years without noticeable problems - I have lived only in an easy climate though.
The gelatine and emulsion coating of the plastic film will have the same moisture content as it's surroundings, however if the humidity is very high then the film may not dry out enough to withstand repeated handling. If this seems to be the case in future then it will be worth looking at using a hardening-fixer to toughen the emulsion chemically. This is a perfectly legitimate way to cope with climatic conditions and the only (slight) downsides are a few more minutes fixing and washing time.